1. Field of the Invention
The present invention relates to a semiconductor device, a charge pumping circuit, and a semiconductor memory circuit.
2. Related Art
A MOS-type capacitor is often adopted as a capacitor element used in a circuit which is fabricated by a CMOS (Complementary Metal Oxide Semiconductor) process, in consideration of matching of the capacitor element to other MOS transistors. A MOS-type capacitor can also be used as an anti-fuse element which performs a write operation by breaking down a gate insulating film and short-circuiting a gate and a substrate.
There are two typical structures for MOS-type capacitors. One is a MOS transistor structure composed of a P well region, two N+ diffusion layers which are formed in the P well region and serve as source and drain electrodes, a P+ diffusion layer which serves as an electrode for the P well region, a gate insulating film which is provided between the N+ diffusion layers on the P well region, and a gate electrode which is provided on the gate insulating film and is made of polysilicon doped with impurities exhibiting N-type conductivity.
When a positive voltage is applied to a gate electrode of such a MOS transistor, and a negative voltage is applied to N+ diffusion layers and a P+ diffusion layer, if the gate voltage is not less than the threshold voltage of the transistor, a channel region is formed between the N+ diffusion layers, and a portion between the gate electrode and the channel region (the gate insulating film) acts as a capacitance. This causes the MOS transistor to function as a MOS-type capacitor.
However, if the gate voltage is less than the threshold voltage, a depletion layer is formed at the surface of the P well region in this MOS-type capacitor. In this case, a depletion layer capacitance generated in the depletion layer and a capacitance in a gate insulating film may be series-connected, and the total capacity of the MOS-type capacitor may decrease.
The other typical structure for MOS-type capacitors is composed of an N well region, two N+ diffusion layers which serve as electrodes for the N well region, a gate insulating film which is provided between the N+ diffusion layers on the N well region, and a gate electrode which is provided on the gate insulating film and is made of polysilicon doped with impurities exhibiting N-type conductivity.
When a positive voltage is applied to a gate electrode of such a MOS-type capacitor, and a negative voltage is applied to N+ diffusion layers, the MOS-type capacitor enters an accumulation state in which negative charges are accumulated between the N+ diffusion layers at the surface of an N well region on the side of a gate insulating film. Even if the voltage is low, the MOS-type capacitor can achieve a relatively large capacity.
However, since the gate electrode of the MOS-type capacitor is doped with impurities exhibiting N-type conductivity, electrons floating in the gate electrode are attracted to the positive charge side (the side opposite to the gate insulating film), and a depletion layer is generated at the surface on the gate insulating film side of the gate electrode. For this reason, a depletion layer capacitance generated in the depletion layer and a capacitance in the gate insulating film may be series-connected, and the total capacity of the MOS-type capacitor may decrease.
An anti-fuse element using a MOS-type capacitor causes an electrical breakdown by applying a high voltage to a gate insulating film. Generation of a depletion layer brings the anti-fuse element to the same state as that when the thickness of the gate insulating film becomes larger, and an applied voltage required for an electrical breakdown increases. Also, the area of a circuit for generating the voltage increases.
A MOS-type capacitor is often used in, e.g., a charge pumping circuit which forms a boosted voltage power supply. A charge pumping circuit is a circuit which boosts the voltage from a low voltage power supply by charge and discharge of a capacitor and supplies a high voltage to an internal circuit. Since a MOS-type capacitor as described above decreases in capacity due to generation of a depletion layer, the voltage boosting capability when it is used in a charge pump is limited.